There is a memory device in which memory cells are three-dimensionally aligned by forming a memory hole in a stacked body where a plurality of electrode layers functioning as control gates in the memory cells and a plurality of insulating layers are alternately stacked, forming a charge storage film on a side wall of the memory hole, and then providing silicon serving as a channel in the memory hole.
In this type of memory device, there is a U-shaped memory string structure that includes: a pair of columnar parts extending in a stacking direction of the stacked body including a plurality of electrode layers; and a coupling part embedded in a back gate and connecting the pair of columnar parts.
Before the formation of the stacked body including the electrode layers, a concave portion (a space portion) is formed in a foundation layer serving as the back gate, and a sacrifice film is embedded in the concave portion. After that, the stacked body including the electrode layers is formed on the foundation layer, then the holes in the columnar parts are formed, and the sacrifice film within the concave portion is removed by etching through the holes. That is, the formation of the U-shaped memory holes includes a process of forming the concave portion and a process of embedding the sacrifice film within the concave portion. After that, the charge storage films and channel body layers are formed within the holes and the concave portion, and thus a nonvolatile semiconductor memory device is formed. This type of nonvolatile semiconductor memory device becomes increasingly smaller. Therefore, in the formation of the memory cells, it is necessary to further enhance the processing accuracy in a photolithography process.